Make sure you are not using any cookie blockers.
Clear your browser's cache.
If using Chrome do a Ctrl+F5 on the main Steemvoter login page. This is a more thorough way to flush the cache.
Make sure all plug-ins are disabled
Make sure you don't have any other third party software that may be interfering with the normal operation of your browser
Try it from a smartphone if nothing else works; the Steemvoter site isn't a great mobile experience but it does work that way.
If you do get it to work, please let us know what you did to fix it as it may be useful to other people who encounter the same problem!